1. Field of the Invention
This invention relates to an integrally cast tissue ingrowth surface apparatus and a method for casting the same. More particularly, the invention relates to an integrally cast bone or tissue ingrowth surface in a cast metal orthopaedic prosthesis.
2. Description of the Prior Art
Investment casting or the "lost wax process" has been used for over 50 years in the production of medical and dental implants. The process derives its name from the investment of wax or other suitable mold material and ceramics used to produce an expendable mold for casting.
The investment casting process used to produce orthopaedic implants is common to the industry and is used to produce implants from Co--Cr--Mo ("Vitallium") alloys as well as titanium alloys and stainless steels. Being well suited to the manufacture of the complex shapes typical of many implant designs, investment casting is used extensively to produce components for bone implants or total joint prostheses such as knees and hips.
These prostheses typically consist of metallic and polymeric components where the metallic components rest against bone on one side of the joint and bear against the polymeric component on the other. The bearing surfaces of a total joint have evolved in design to closely mimic the movement of the natural joint, while the bone contacting sides have evolved to assure improved fixation of the implanted prosthesis with the surrounding bone.
Until recently, total joint prostheses were designed for implantation with bone cement. For example, a polymethylmethacrylate (PMMA) grouting agent may be used to secure the prosthesis component against the surrounding bone. Implant surfaces contacting the cement were either cast smooth or with a two dimensional texture intended to improve fixation with the PMMA grout.
Recurrent loosening of these cemented implants, due to loss of support in underlying bone, lead to the development of prostheses with three dimensionally porous fixation surfaces which could be used without the PMMA bone cement. These prostheses instead rely on fixation via the ingrowth of bone or other connective tissue directly into the prosthesis surfaces, thereby anchoring the prosthesis to the bone.
These three dimensionally textured surfaces are created by bonding a suitable network of material, usually metal of the same composition as the implant, onto the implant's fixation surfaces to create a porous coating. The nature of the porosity present in the coating is generally a direct function of the materials and methods used to produce the coating.
Porous surfaces have been created by plasma spraying (U.S. Pat. No. 3,605,123) of fine metallic particles, or by sintering a loosely packed coating of metallic particles (U.S. Pat. No. 4,550,448, British Patent 1316809), or by diffusion bonding kinked fiber metal pads (U.S. Pat. No. 3,906,550), or overlapping mesh (U.S. Pat. No. 4,636,219).
In another concept, integrally formed ceramic filled porous areas are formed on the prosthesis. U.S. Pat. No. 4,722,870 discloses a method for investment casting a composite implant which produces a porous metal structure filled with a ceramic (hydroxyapatite). However, this structure cannot be accurately controlled nor can it be spaced a predetermined distance above the outer surface of the implant.
Other U.S. patents describe mesh surfaces welded to the implant. Such a mesh is shown in U.S. Pat. No. 3,905,777 to Lacroix, U.S. Pat. No. 4,089,071 to Kalnberz et al, U.S. Pat. No. 4,261,063 to Blanquaert and U.S. Pat. No. 4,636,219 to Pratt et al. None of these surfaces are integrally cast with the prosthesis.
Each of the aforementioned methods for producing a porous ingrowth surface entails applying a porous network onto the surface of a metallic implant and bonding that network through the application of heat. Plasma spraying employs super heated gases to melt the metal particles to be sprayed. Sintering develops interparticle bonds in a porous coating by exposing the coating and implant metal to temperatures approaching their melting point, while diffusion bonding employs heat and pressure to promote atomic diffusion at the coating implant interface.
Each of these methods has its limitations. Plasma spraying cannot be adequately controlled to achieve a uniform interconnected pore structure in the coating. The temperatures required for sintering have a deleterious effect on the implant materials strength, and diffusion bonding develops variations in pore structure and bond quality due to variations in pressure distribution during the coating process. Each of the processes is limited in its achievable pore size by the loss in coating strength which occurs as coating porosity increases.
Particulate porous coatings are also inherently accompanied by a dramatic increase in surface area of metal exposed to body fluids, increasing proportionally the corrosion products which are released after implantation.
Clinical reports exist of metal particles becoming loose from bonded coatings or fiber pads becoming detached on revision surgery. Furthermore, bonded coatings inherently develop stress concentrating surface notches at the coating - substrate interface which limit the locations a porous coating can be placed due to strength considerations. By their very nature, bonded coatings require the use of a secondary manufacturing process to affix the coating to the implant surfaces. These processes increase manufacturing costs through added labor, materials, tooling and fixturing.